426 JENNINGS— HEREDITY IN PROTOZOA. [April 24, 



microns, while the posterior half was shorter, but thicker, measuring 

 96 X 52 microns. The succeeding changes of form will be described 

 in the next section. 



Thus from the condition shown at d, Fig. 3, to the completion 

 of fission a period of twenty-one minutes elapsed. From the earliest 

 appearance of the constriction the time till separation is usually a 

 little more than one half hour. 



Second Stage: the Young Immediately after Fission up to the Age 

 of Ninety Minutes. 



Observation of Living Specimens. — Immediately after separation 

 of the two halves, growth occurs rapidly, and the shape changes, both 

 halves becoming more pointed at both ends. In the specimens of the 

 aurelia form under description at the close of the last section, the 

 posterior half had two minutes after fission increased in size from 

 96 X 52 microns to 104 X 48 microns. Eight minutes after separa- 

 tion both halves measured 112 microns in length, so that they had 

 during that period increased respectively 12 and 16 microns in length. 

 The difference between anterior and posterior individuals was still 

 marked. 



Now followed a period of slower growth. At 12.53, twenty- 

 seven minutes after division, each half measured approximately 120 

 microns in length. They had taken nearly the characteristic adult 

 form and it was no longer possible to distinguish the anterior product 

 from the posterior one. 



At 2 P. M. (one hour and thirty-four minutes after separation) 

 the length was about 135 microns and the progeny were similar to 

 the adult specimens of the aurelia form. 



Thus, at the time of separation the two individuals have some- 

 what more than half the adult length ; they grow rapidly at first, then 

 slowly, and in an hour and a half have reached nearly the adult size. 

 (As later statistical studies show, growth continues for a long time 

 still.) 



Observation on the growth of living specimens of the caudatuni 

 form gave a parallel series of phenomena (see Fig. 4). Thus, in a 

 descendant of D, the length of each half at the time of separation 



